the answer

Reactions occur that are thermally neutral. That is: the products have the same energy as the reactants, BUT there is always an activation barrier (energy hill) between reactants and products. Reactants have to absorb heat to climb the hill and then give it back out on the way down the hill.

I don't know of any reaction that is thermally neutral AND has a zero energy of activation. However, with catalysis--a way for reactions to be facilitated by a third compound which is not changed-- and in particular in biological systems, which use enzymes for catalysis, it is possible to achieve activation energy at normal Earth temperatures. Room temperature offers enough heat to get over an energy hill of 20 kcal/mole.

However, you still don't get something for nothing. Energy exchange is required by the system to make the reaction go. It just "appears" to be nothing because it happens at room temperature.